1. Field of the Invention
The invention relates to a nozzle head for a rotary atomizer for applying a coating material to an object, having                a) a rotary bell which is rotatable about an axis of rotation and has a breakaway edge and a discharge surface to which the coating material can be supplied in such a way that the coating material is spun off from the breakaway edge of the rotary bell, and        b) a flow path via which the coating material can be supplied to the discharge surface.        
The invention moreover relates to a rotary atomizer for applying a coating material to an object with a nozzle head.
2. Description of the Prior Art
Rotary atomizers which are equipped with a nozzle head of the type mentioned at the outset are used for example in the automotive industry to paint objects, such as parts of vehicle bodies, or to coat them with a protective material.
The rotary bell here serves to atomize the coating material, for which it is rotatedabout its axis of rotation at very high rotational speeds of 10,000 to 100,000 rpm by means of a pneumatic or electric drive during operation.
The selected coating material is supplied to the rotating rotary bell. As a result of centrifugal forces which act on the coating material, it is driven outwards to the rotary bell as a film, until it arrives at a radially outer breakaway edge of the rotary bell. Here, high centrifugal forces act on the coating material in such a way that it is spun off tangentially in the form of fine coating-material droplets.
During this, droplets of different sizes are produced, which cover a relatively large size range. Larger droplets are spun radially further outwards than smaller droplets. With nozzle heads and rotary atomizers of the type mentioned at the outset, a relatively wide spray jet is thus generated, which is ideally conical and has a relatively large cone angle.
It is desirable here if the size of the droplets is relatively uniform and if the range on the size-related droplet spectrum is as small as possible. Moreover, the droplets should be as small as possible since a more homogeneous coating result is achieved with smaller droplets. The aim is to generate a so-called paint mist. The term mist generally refers to a mixture of air and finely distributed solid or liquid particles. To ensure that the coating material is applied to the object to be coated, a wetting mist with a minimum droplet size in the range of 20 to 40 μm is required. Good results can be achieved with a mean droplet size of 100 μm, with the deviation ideally being ±50 μm.
The more slowly the rotary bell is rotated, the larger, on average, are the droplets which are spun off from the breakaway edge. Accordingly, droplets generated at the breakaway edge of the rotary bell are, on average, smaller at higher rotational speeds of the rotary bell. For this reason, the rotary bell is generally operated at high speeds, which involves a correspondingly high energy consumption. At the same time, the radial dispersion of the spray jet is in turn greater at higher speeds than at lower speeds, which means that measures have to be implemented to focus this spray jet onto the objects to be coated.
To this end, known rotary atomizers operate for example electrostatically. In this case, the coating material to be applied is charged, whilst the object to be coated is earthed. With this, an electrical field forms between the rotary atomizer and the object, as a result of which the charged coating material is applied in directed manner to the object. However, this only functions in the case of electrically conductive objects.
Alternatively or also in addition to the electrostatic operation, guide air devices have become established in known rotary atomizers. With these, a generally annular guide air flow is conducted onto the spray jet in such a way that this latter is collimated and the different-sized droplets are guided to the object to be coated.However, strong guide air flows are sometimes required for this, the generation of which is relatively complex.
DE 43 30 602 A1 discloses a rotary atomizer for electrostatic coating with a static nozzle assembly, which has a plurality of coating-material nozzles which communicate with corresponding coating-material sources by way of various channels. Each nozzle and the channel connected thereto are used to supply only a specific coating material.
This entails a very complex internal design of the rotary atomizer and therefore increased production costs. There is furthermore a risk that coating material which resides in the individual channel over a relatively long period of time will form deposits on the channel walls. When the channel is used again, these deposits can become loose and lead to an unusable coating result.